When functioning normally, the heart produces rhythmic contractions and is capable of pumping blood throughout the body. The heart has specialized conduction pathways in both the atria and the ventricles that enable excitation impulses (i.e. depolarizations) initiated from the sino-atrial (SA) node to be rapidly conducted throughout the myocardium. These specialized conduction pathways conduct the depolarizations from the SA node to the atrial myocardium, to the atrio-ventricular node, and to the ventricular myocardium to produce a coordinated contraction of both atria and both ventricles.
The conduction pathways synchronize the contractions of the muscle fibers of each chamber as well as the contraction of each atrium or ventricle with the opposite atrium or ventricle. Without the synchronization afforded by the normally functioning specialized conduction pathways, the heart's pumping efficiency is greatly diminished. Patients who exhibit pathology of these conduction pathways can suffer compromised cardiac output.
Cardiac rhythm management devices have been developed that provide pacing stimulation to one or more heart chambers in an attempt to improve the rhythm and coordination of atrial and/or ventricular contractions. Cardiac rhythm management devices typically include circuitry to sense signals from the heart and a pulse generator for providing electrical stimulation to the heart. Leads extending into the patient's heart chamber and/or into veins of the heart are coupled to electrodes that sense the heart's electrical signals and deliver stimulation to the heart in accordance with various therapies for treating cardiac arrhythmias and dysynchronies.
Pacemakers are cardiac rhythm management devices that deliver a series of low energy pace pulses timed to assist the heart in producing a contractile rhythm that maintains cardiac pumping efficiency. Pace pulses may be intermittent or continuous, depending on the needs of the patient. There exist a number of categories of pacemaker devices, with various modes for sensing and pacing one or more heart chambers.
A pace pulse must exceed a minimum energy value, or capture threshold, to “capture” the heart tissue by generating a propagating depolarization wave that results in a contraction of the heart chamber. It is desirable for a pace pulse to have sufficient energy to stimulate capture of the heart chamber without expending energy significantly in excess of the capture threshold. Thus, accurate determination of the capture threshold is required for efficient pace energy management. If the pace pulse energy is too low, the pace pulses may not reliably produce a contractile response in the heart chamber and may result in ineffective pacing. If the pace pulse energy is too high, the patient may experience discomfort and the battery life of the device will be shorter.
Detecting if a pacing pulse captures the heart allows the pacemaker to adjust the energy level of pace pulses to correspond to the optimum energy expenditure that reliably produces capture. Further, capture detection allows the pacemaker to initiate a back-up pulse at a higher energy level whenever a pace pulse does not produce a contraction.
Devices for cardiac pacing and sensing may utilize a number of electrodes electrically coupled to the heart and configured to sense and/or pace a heart chamber. Pacing via multiple intra-chamber electrode pairs may be beneficial, for example, to stimulate the heart tissue in a coordinated sequence that improves contractile function of the heart chamber. It is desirable to determine the energy required to capture the heart tissue for each set of electrodes used for pacing. The present invention provides methods and systems for estimating capture thresholds and provides various advantages over the prior art.